Climbing with "double jointed" fingers

Hello MP, I have searched around looking for advice on this but haven't found much. I have what a lot of people call double joined fingers (the tendon slips off the side of my knuckle) and I find it effects my climbing especially on crimps. I think I have noticed it slowly improve as I climb more, but I am curious to hear if any of you have struggled with the same issue/what you did to help. One thing I was thinking were band assisted open crimp hangs

No personal experience but the Nugget episode with Paul Houghoughi talks about hyper mobility and recommendations to deal with it. Would probably be worth reading on what sort of climbing and hangboarding will make your fingers stiffer too, several resources out there or just go straight to the source and use NCBI or Google Scholar to find scientific study abstracts.

I used to be extremely flexible in my wrists and fingers (I could touch the back of my forearm with my fingertips pushing my hand backwards)

Not sure exactly when in my climbing journey I stopped being able to do that but I'm obviously much stronger now and cant do that anymore.  You're going to tighten up in your hands and fingers as you go, and I'm sure it wont be an issue. 

Taking a cursory look at your tick list and how long you've been climbing, I promise your fingers arent used to crimping all that hard yet.  Give it time, sooner or later you'll be much more worried about loosening up your fingers to get them warm than getting them tighter...